Subject matter is the starting point for your art. What will you be "talking" about in the piece?
Subject matter is the broad theme or topic of your piece. For example, perhaps my subject matter is "horse". This is the subject. This is the starting point.
Content is very closely related to your subject matter, and often we slide from one to the other without being aware of it. Content can be seen as a refinement of subject matter. Once you have determined your subject matter, now it is time to define what it is you want to say on that topic.
Content is the foundation of your art in which you clearly define what it is you are attempting to communicate about your subject matter. What concept or set of interrelated concepts will you be working to impart?
Using our horse example from before, it may be that my content will be "horse [subject matter] as a symbol for freedom and strength [content]".
Form is the culminating foundation in 3-D art. Without form, there is no expression of subject or of content. Form is the physical manifestation of your artistic vision, expression, and communication. For 3-D artists, form is much more involved than for 2-D artists, such as painters.
This is true because, unlike a 2-D artwork, the 3-D artwork must serve the subject matter and the content from a multiplicity of viewpoints. No matter how the audience moves around the form, the artwork needs to continue to do its job of speaking to that audience.
Form and Surface in Clay
When working with clay, form has the ability to become an even richer medium for the artist and experience for the audience. Not only do we as potters and ceramists work with the 3-D form, but also the 2-D possibilities that exist in decorating or enhancing our works' surface.
When developing the piece, remember that the surface is an integral part of the overall piece. Surface treatments need to mirror all the foundations in order to present a unified, complete three dimensional artwork, including functional ceramic work.
In your own pottery, whether it is functional or sculptural, take the time to consciously evaluate your own work's subject matter, content, and form (including surface).